i leave the helicopter we're supposed to take to jump from a black hawk to another one that fly's straight to the building. I do a grenade boost to get onto the roof. Because I got to the hostages so fast the helicopters stay still outside. When i get the objective to get on the 50 cal i hit a trigger just before which makes the black hawk that gets shot down at the end of the mission turn from green to blue on the map. When it's blue it can be destroyed at any time. So i use an m60 to destroy it almost instantly.